1. Field of the Invention
The present invention generally relates to medical devices and more specifically relates to surgically implantable electrodes.
2. Description of the Prior Art
The earliest cardiac pacers involved electrical stimulation of a single chamber (i.e., right ventricle) of the heart. Indeed, this is still the most prevalent technique. However, a number of medical conditions are more effectively treated using stimulation to two chambers (i.e., right atrium and right ventricle) or using a combination of sensing in one chamber and stimulation in another.
The desired dual chamber pacing (i.e., by an electrode in each of the right atrium and the right ventricle) is most commonly accomplished using two separate leads. This is relatively convenient for epicardial applications, but may be a problem for transvenous insertion of electrodes. Some techniques are two veins whereas others involve insertion of two leads in a single vein. However, a single pass dual chamber lead is in demand.
An early single pass A-V (atrial and ventricular) lead was taught by Bures in U.S. Pat. No. 3,865,118. Because the configuration taught by Bures requires the ventricular lead to be coaxially mounted within the outer sheath, minimal control can be exercised over placement of the atrial electrodes. To compensate for this lack of control, Bures teaches the use of opposing (i.e., spaced by 180.degree.) spring loaded electrodes. Such a placement technique has been shown to be susceptible to dislodgement and is electrically inefficient because of the relatively large surface area of the electrode and the difficulty in controlling the amount of that surface area actually in contact with the atrial wall. Furthermore, using the outer catheter to control flexure of the atrial electrodes has lead to sealing problems. The alternative embodiment of FIG. 5 overcomes a number of these problems by attaching the atrial electrodes directly to the outer catheter. Unfortunately this configuration is electrically very inefficient because contact between the atrial electrodes and the endocardium cannot be assured chronically.
Lajos in U.S. Pat. No. 4,057,067 solves many of the control problems found with the lead taught by Bures by using a "J" shaped atrial lead with stylet control. However, because the atrial and ventricular leads are spaced at a fixed distance, the lead taught by Lajos does not accommodate hearts having varying distances between the right ventricular apex and the right atrial appendage which are the most desired sites for location of the electrodes. A further problem with the Lajos lead is the establishment of an effective seal of the hole at the distal end of the atrial electrode. During insertion, this hole is blocked by the stylet. However, removal of the stylet causes seepage of blood into the lead.
A third single pass lead configuration is taught by Sabel in U.S. Pat. No. 3,949,757. Sabel uses the "J" shaped atrial electrode placement as taught by Lajos but slides the atrial catheter within the outer sheath of the ventricular catheter. This tends to solve one problem of Lajos but not requiring an aperture in the distal end of the atrial electrode for stylet straightening of the "J" shape. It does not completely solve the problem of differing heart sizes, however. The distance between the distal end of the atrial catheter and the distal end of the outer sheath 28 is essentially fixed by practical factors even though the atrial catheter is slideably mounted within outer sheath 28, because sliding of the atrial catheter also changes the shape of the "J". This may be viewed in FIG. 7. The atrial electrode may be lowered in the atrium by moving the atrial catheter either proximal or distal relative to the outer sheath. However, the atrial electrode may not be raised within the atrium. That distance is effectively established by the prior implantation of the ventricular electrode. Providing a larger distance between the ventricular electrode and the distal end of outer sheath 28 would tend to distort the "J" shape of the atrial catheter.